Recently, titanium oxide photocatalysts have been put into practical use in various situations, for the purposes of sterilization, antifouling, and the like. The use of the same now is not limited to outdoor use, but is spreading to indoor use for the purposes of sterilization, deodorization, and the like. Because of this, a titanium oxide has been demanded that can be excited efficiently even by an energy in a visible region in a titanium oxide excitation system that conventionally has required an energy in an ultraviolet region. Such demand often is met by a titanium oxide supporting a foreign element or forming a solid solution with a foreign element. A wavelength for exciting the titanium oxide can be controlled depending on the type of a foreign element to be added.
However, in many cases, such a treatment that causes titanium oxide to support a foreign element or causes titanium oxide to form a solid solution with a foreign element significantly reduces an efficiency of excitation inherent to the titanium oxide. In return for the excitability with respect to visible light, an effect to be achieved originally by ultraviolet rays is reduced, which results in a decrease in activity in many cases.
Conventionally, it is known that the photocatalytic activity of titanium oxide is enhanced by elimination of lattice defects in titanium oxide using a mineral acid or the like (Non-Patent Document 1). Especially, it is known that a hydroxyl group on a surface of titanium oxide can be replaced easily with fluorine. Therefore, there have been proposals to treat titanium oxide with a fluorine compound such as hydrofluoric acid so as to enhance the photocatalytic performance in the titanium oxide excitation system using ultraviolet rays (Non-Patent Document 2, Patent Documents 1 and 2). However, some types of titanium oxide treated as above did not fully exhibit the effect.
On the other hand, regarding deodorization and purification of air, a technology that is capable of promptly deodorizing and decomposing four major odorous components—acetaldehyde, acetic acid, ammonia and sulfur compound gas (e.g. hydrogen sulfide and methyl mercaptan)—has been demanded. Exemplary methods of the above technology are as follows: a method of concentrating and storing odor using an adsorbent such as activated carbon or zeolite; and a method of directly decomposing odor by thermal decomposition, thermal catalytic decomposition, ozone decomposition, plasma discharge decomposition, photocatalyst decomposition, or the like.    [Patent Document 1] JP 07-303835 A    [Patent Document 2] JP 2004-292225 A    [Non-Patent Document 1] Proceedings Electrochemical Society 1988, vol. 88, no. 14, pp. 23-33    [Non-Patent Document 2] The Journal of Physical Chemistry 1990, vol. 94, pp. 4276-4280